1. Field of the Invention
The present invention relates to an electroluminescence display device comprising an electroluminescence element and thin film transistors.
2. Description of Related Art
An electroluminescence (EL) display devices using an electroluminescence element have recently attracted interest as potential replacements for devices such as CRT or LCD displays. For example, an EL display device having a thin film transistor (TFT) as a switching element for driving the EL element has been studied and developed.
FIG. 1 shows an equivalent circuit diagram of an organic EL display device comprising an EL element and TFTs according to a related art. The organic EL display device is shown in a plan view in FIG. 2 and in cross sections taken along line Axe2x80x94A in FIG. 3A and along line Bxe2x80x94B in FIG. 3B, respectively.
The circuit in FIG. 1 comprises a first TFT 130, a second TFT 140, and an organic EL element 160. Shown in the figure is a region around where a gate signal line Gn in the n-th row is orthogonal to a drain signal line Dm in the m-th column, and in the vicinity of the cross section of both signal lines are provided the organic EL element 160, and the TFTs 130, 140 for driving the organic EL element 160.
The first TFT 130, which is a switching TFT, comprises gate electrodes 131 connected to the gate signal line Gn and having a gate signal supplied thereto, a drain electrode 132 connected to the drain signal line Dm and having a drain signal supplied thereto, and a source electrode 133 connected to a gate electrode 141 of the second TFT 140.
The second TFT 140, which is an organic EL element driving TFT, comprises a gate electrode 141 connected to the source electrode 133 of the first TFT 130, a source electrode 142 connected to an anode 161 of the organic EL element 160, and a drain electrode 143 connected to a power supply 150 for the organic EL element 160.
The organic EL element 160 comprises the anode 161 connected to the source electrode 142 of the second TFT 140, a cathode 162 connected to a common electrode 164, and an emissive element layer 163 interposed between the anode 161 and the cathode 162.
A capacitor 170 is further provided with one electrode 171 being connected between the source electrode 133 of the first TFT 130 and the gate electrode 141 of the second TFT 140 while the other electrode 172 being connected to the common power supply 150.
When a gate signal is applied to the gates 131 of the first TFT 130, a drain signal is applied from the drain 132 to the source 133 and is further applied to the storage capacitor 170 and the gate 141 of the second TFT 140.
When a voltage is applied to the gate 141, an electrical current in accordance with the voltage is supplied from the power supply 150 to the organic EL element 160 which then emits light.
In the conventional EL display device as described above, the characteristics of the TFT 140 for supplying a current to drive the organic EL element 160 vary in each display pixel. For example, if the second TFT 140 has an active layer comprising a polycrystalline semiconductor layer formed by irradiating a laser to an amorphous semiconductor layer, the laser light is not uniformly irradiated in the channel region of each semiconductor layer. As a result, the grain size of is crystal in the semiconductor layers becomes non-uniform, and the characteristics, such as on-state current, vary.
If the characteristics vary in the second TFT comprising a single TFT as described above, the current values to be supplied to the organic EL element 160 also vary in each display pixel, which in turn leads to non-uniform luminance of the display pixels.
The present invention is made in view of the aforementioned problems of the related art and aims to provide an EL display device which can achieve display with uniform luminance in each display pixel, even if the characteristics of TFTs vary in the display pixels for driving the EL element.
In one aspect of the present invention, there is provided an electroluminescence (EL) display device comprising an EL element having an anode, a cathode, and an emissive layer interposed between the anode and the cathode; a first transistor formed by a thin film transistor in which a drain of an active layer using a non-single crystal semiconductor film is connected to a drain signal line and gate electrodes provided above or under channels of said active layer are connected to a gate signal line; and a second transistor formed by a plurality of thin film transistors in each of which a drain of an active layer using a non-single crystal semiconductor film is connected to a drive souse of said EL element and gate electrodes are respectively connected in parallel with the source of the first thin film transistor.
In accordance with another aspect of the present invention, there is provided an electroluminescence device comprising an EL element having first and second electrodes and an emissive layer provided between the first and second electrodes; a switching thin film transistor which operates when a gate thereof receives a gate signal and captures a data signal; and an element driving thin film transistor provided between a power supply and the EL element for controlling power supplied from the power supply to the EL element according to the data signal supplied from the switching thin film transistor, said element driving thin film transistor being constituted by a plurality of thin film transistors connected in parallel to each other.
As described above, a plurality of thin film transistors are connected in parallel for supplying power from the power source to drive the EL element, such that the effects of variance in the characteristics of each of the thin film transistors upon the entire characteristics for the set of thin film transistors can be reduced, even when the characteristics vary in individual transistors. Thus, electrical power can be stably supplied by the plurality of thin film transistors to the EL element which generates luminance depending on the power amount (current amount) supplied from the power source, thereby suppressing the variance in luminance of the EL element.
When said EL element, and said switching (first) and element driving (second) thin film transistors constitute one pixel, and a display device is comprised of a plurality of pixels arranged in matrix on a substrate, the present invention can effectively prevent the luminance from varying in each pixel to achieve uniform display.
In accordance with another aspect of the present invention, the active layer of each thin film transistor of the first and second transistors or the active layer in each of the switching thin film transistor and the element driving thin film transistor comprises a non-single crystal semiconductor layer.
The non-single crystal semiconductor layer may be, for example, a polysilicon layer.
Further, in accordance with still another aspect of the present invention, the active layers in respective thin film transistors of the first and second transistors or the active layers in the switching thin film transistor and the element driving thin film transistor are non-single crystal semiconductor layers formed in the same process step.
By forming the active layers of these transistors in the same process, no extra process steps are required, even if a structure is adopted in which a plurality of thin film transistors constituting the element driving thin film transistor are connected in parallel.
In accordance with another aspect of the present invention, a storage capacitor is further provided between the source of the switching thin film transistor and each gate of a plurality of thin film transistors constituting the element driving one thin film transistor.
The storage capacitor simplifies the holding of data obtained when the switching thin film transistor, which is selected every predetermined period, is selected, until next time the switching thin film transistor is selected, thereby improving the display quality of the display device.
In accordance with still another aspect of the present invention, the electroluminescence element is an organic electroluminescence element having an emissive layer which uses an organic compound.
An emissive layer using an organic compound is very advantageous in a color display device due to the variation of emissive colors and a wide selection of possible materials.
In accordance with another aspect of the present invention, the switching and element driving thin film transistors have a top gate or bottom gate structure in which the gate electrodes are disposed above or under the active layer of each transistor.
In accordance with still another aspect of the present invention, the switching thin film transistor has a Lightly Doped Drain structure and/or multigate structure. Such a structure will allow improved rapid response and reduced leak current at the off mode.